This invention relates to computerized tomography (CT) systems, and more particularly to communication of high data rate signal images across the CT rotating interface.
As known, CT systems are used to obtain non-invasive sectional images of test objects. The most common use is to provide internal images of human patients for medical analysis and treatment. In operation, the object or patient is positioned on a table within a central aperture of a rotating frame, or gantry, which is supported within a stationary frame. The gantry includes an x-ray source and a detector array positioned on opposite sides of the aperture, within the system's imaging plane, and each rotate with the gantry around the object being imaged. At each of several angular positions along the rotational path the x-ray source emits a collimated beam which passes through the object and is received by the detector array. Sensors within the detector array produce electrical signal indications of the x-ray intensity incident at their surface, and these signals are collated by circuitry within the rotating frame into a set of image data at each angle. Each image data set is referred to as a view, and the plurality of views taken in each revolution, referred to as a scan, are processed by a stationary side computer into a cross sectional image of the object.
It is known to transfer detector data across the rotating gantry interface to the stationary side computer using a non-contact, electromagnetic coupling referred to as an RF (radio frequency) slipring. The data transfer occurs during scanning. There are nominally 1000 views in a full (360.degree.) scan, and a typical maximum gantry slew rate of 360.degree. per second. For a CT system with 752 detector channels, each channel providing a data signal with 16 bit image resolution, the data signal bit speed=(752.times.1000.times.16)/1.0=12.03 Mbps. The bit cell time is 83 nano seconds. This is a comparatively slow bit rate, with a corresponding long interval bit cell time, which minimizes the affects of ambient noise on the signal integrity. In these systems the data is amplitude modulated with an RF carrier signal and transmitted through the RF slipring to the stationary side.
In newer CT systems, for reasons related to patient comfort and efficiency, there is an emphasis on reducing the time spent in performing CT scans. This has led to CT designs capable of producing multiple slice images within a single rotation. One such proposed CT system produces four slices per revolution and a gantry slew rate of 720.degree. per second, or 0.5 seconds per revolution. With the same 16 bit element signal resolution and 1000 images per slice per revolution, the resulting data rate is: (752.times.4.times.1000.times.16)/0.5=96.26 Mbps. With the addition of overhead bits the signal bit rate approaches 110 Mbps with a bit cell time of 9.2 nano seconds. This is nearly an order of magnitude increase in required throughput across the rotating interface.
While RF amplitude modulation is cost effective compared with alternative modulation methods, it is noise susceptible. As the data signal bit speed increases, ambient noise has an increasingly greater effect due to the smaller bit cell times. This reduced cell time causes the data bits to be increasingly susceptible to induced noise, including the loss of or displacement of the data bits, so as to lose synchronization and cause "jitter" in the data stream. It is currently known to use forward error correction (FEC) of the data stream to reduce this noise and jitter, however, FEC is expensive and complex to implement.